One common method of interconnecting multiple local area networks (LANs) is via one or more media access control (MAC) bridges. The bridged LAN that is generated as a result of such interconnection allows end devices attached to separate LANs to communicate with each other as if they were attached to a single LAN. IEEE Standard 802.1D entitled “IEEE Standard for Information Technology—Telecommunications and Information Exchange Between Systems—IEEE Standard for Local and Metropolitan Area Networks—Common Specifications—Media Access Control (MAC) Bridges,” 1998 Edition, the content of which is incorporated herein by reference, specifies a general method for the operation of such MAC bridges.
Bridged LANs may be extended to support virtual local area networks (VLANs). In virtual bridged LANs, data units (e.g. packets or frames) are classified into virtual LANs (VLANs) in order to differentiate service within a bridged LAN. IEEE Draft standard 802.1Q entitled “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridge Local Area Networks,” 1998, and IEEE Draft Standard 802.1V entitled “IEEE Standard for Local and Metropolitan Area Networks—Virtual Bridged Local Area Networks—Amendment 2: VLAN Classification by Protocol and Port,” 2001, the contents of which are hereby incorporated by reference, provide standard VLAN classification rules.
In general terms, IEEE 802.1Q provides VLAN tagging rules for optionally adding a tag header including an assigned VLAN identifier (ID) to a data unit prior to transmitting it on an egress port. Tags are applied or not depending on the VLAN of the data unit. The egress port tags the data unit unless the VLAN of the data unit belongs to an untagged set of VLANs. In this instance, the data unit is transmitted without the VLAN tag header via an untagged egress port.
Bridges in a virtual bridged LAN may include switch ports configured as non-mobile or mobile ports. Non-mobile ports may be assigned to a default VLAN for switching untagged traffic. Generally only one default VLAN may be authorized per port.
In order to support more than one VLAN per port, a network administrator may choose to tag a particular port with one or more VLAN identifiers. In this case, the bridge switches tagged traffic with a VLAN identifier matching a VLAN identifier tagged to the port. One drawback with tagged ports, however, is that the network administrator must generally know in advance the traffic that will be flowing through the tagged ports in order to tag the ports in an appropriate manner.
Mobile ports were thus designed to overcome this deficiency. One of the benefits of mobile ports is that they may be dynamically associated with different VLANs based on the traffic flowing through them. In this regard, mobile ports are generally configured with one or more VLAN rules that are examined for a match upon receipt of traffic through the port. If the traffic matches a VLAN rule configured on the port, the port becomes a member of that VLAN. If the traffic does not match a configured VLAN rule, the mobile port remains associated with a default VLAN. The mobile port may also belong to more than one VLAN if the port carries traffic, that is, different flows, matching different VLAN rules.
Once the VLAN rules have been configured on the mobile ports, mobile devices may be moved transparently in a given set of mobile ports, where the same VLAN rules apply on that set of ports. Such dynamic configuration, however, generally requires that the mobile device transmit traffic in order to be classified into a desired VLAN. Silent mobile devices such as, for example, printers, do not generally transmit traffic that will allow the VLAN classification. Thus, when a silent mobile device is connected to a mobile port, the mobile port remains associated to its default VLAN, and no dynamic association of the port to the VLAN of the attached device occurs. As a result, unless the silent mobile device is a member of the default VLAN, it remains unreachable to the other devices given that the port does not forward traffic on a VLAN different than the VLAN to which it is associated.
Accordingly, what is desired is a system and method for dynamically associating a mobile port based on the VLAN membership of a device connected to the port, even when the device does not initiate traffic that may be examined by the port to allow such dynamic association via traditional mechanisms.